The purpose of this project is to determine the mechanism by which senescent human red blood cells (RBC) and RBC stored in vitro for transfusion are removed from the circulation. Experiments performed in this laboratory suggest that macrophages distinguish autologous senescent from mature RBC on the basis of selective IgG attachment to the former. The Ig eluted from senescent cells and RBC stored as whole blood is an IgG free of other Ig's as determined by immunodiffusion, immunoelectrophoresis and polyacrylamide gel electrophoresis. The IgG eluted from senscent cells reattaches to homologous cells by the Fab region and initiates their phagocytosis by macrophages. Methods for separating macrophages from human peripheral blood and mouse spleen cell suspensions have been developed. Isolated murine macrophages phagocytize senescent but not young RBC in vitro, and murine splenic macrophages phagocytize 59Fe labeled old but not young RBC in situ. Thus, the IgG which attaches to senescent cells in situ appears to be an autoantibody which contributes to the maintenance of homeostasis by initiating the removal of senescent, stored and damaged cells. Experiments suggest that this IgG may limit the transfusion effectiveness of blood cells by binding to them during storage.